1. Field of the Invention
The present invention relates to a process for converting heavy hydrocarbon oils into light hydrocarbon fuels. It particularly relates to an improved thermal cracking or hydrocracking process which produces less carbonaceous matter during operation.
2. Background Art
Thermal cracking or hydrocracking has been practiced as a major process for converting heavy oils into light fuels; however, a disadvantage for the process is a marked carbon formation during operation and the relatively low yield of liquid product.
Although thermal cracking process is conducted at a moderate pressure and is not expensive, it fails to carry out a long-term continuous operation and is difficult to obtain high conversions due to the marked carbon deposition; therefore, the yields of distillate fuels are low.
When hydrocracking is employed in the conversion of asphaltene-containing heavy oils, it also fails to carry out a stable continuous operation because of large pressure drop due to carbon deposition. Moreover, the operating conditions and continuous operating duration are limited by a sharp decrease in catalytic activity due to increased carbon formation.
Accordingly, it is a principal object of the present invention to provide an improved process for converting heavy hydrocarbon oils by thermal cracking or hydrocracking into light hydrocarbon fuels wherein the process can increase the continuous operating duration and the yields of liquid products by decreasing the foregoing carbon deposition and thereby setting up more profitable operating conditions.
It is known in the art that a hydrogen-donor substance may be added to the reaction zone in thermal cracking or hydrocracking of heavy hydrocarbon oils in order to prevent the carbon deposition. Oil & Gas Journal, VOL. 85, NO. 28, pp. 84-87, 1987 for example, disclosed a catalytic cracking process wherein at least part of cracked products is hydrogenated and the resulting hydrogenated product is added to the reaction zone. However, in conventional processes a great amount of additive is required because the conventional additive does not have a hydrogen donating ability in a large enough amount. The amount of hydrogen donor substance to be added usually amounts to 30% to several times the amount of heavy hydrocarbon feedstock, on a weight basis.
The present invention is based on a discovery that a good hydrogen-donor substance can be obtained by aromatic ring hydrogenation of a petroleum fraction, and by addition of the substance at reduced amounts, an improved inhibitory effect on carbon deposition can be produced when compared to those of a conventional hydrogen-donor substance.